If you weren't already convinced that the government can listen in on your cellphone whenever it wants, you will be now. An internal NSA document has revealed that the agency can easily crack the most widely used cellphone encryption in the world.

The encryption is called A5/1 and is used by most cellphones around the world. Usually, someone would need an encryption key to unscramble these calls, but the NSA can listen in without one. And, if the NSA can crack the code and listen to your calls, it's likely other countries' security agencies can, too.

Experts say the agency may also be able to decode newer forms of encryption, but only with a much heavier investment in time and computing power, making mass surveillance of cellphone conversations less practical.

2G, or second generation, networks are most vulnerable to this kind of hacking. It's the type of network used by most cellphones in the world, but many cell users in the U.S. now have access to 3G and 4G networks that have better encryption.

But even where such updated networks are available, they are not always used, because many phones often still rely on 2G networks to make or receive calls.

Certain companies like Verizon and Sprint use a newer technology called CDMA, but it's not clear whether the NSA can crack this code or not.